The invention relates to a system for refractive ophthalmological surgery.
In refractive ophthalmological surgery the optical refractive properties of the eye, particularly of the cornea, are changed by using laser radiation in order to correct or at least alleviate visual deficiencies. A prominent example of refractive ophthalmological surgery is LASIK, in which corneal tissue is resected (ablated) in order to reshape the cornea for the purpose of correcting sight defects. For the purpose of resecting corneal tissue, as a rule an excimer laser in the UV region (typically 193 nm) is employed. The laser radiation is guided over the eye in such a way with respect to time and location that a certain amount of tissue is resected at selected points in the cornea. This resection is described by the so-called ablation profile, i.e. the ablation profile specifies the resection (ablation) to be performed at each point in the cornea.
The present invention relates, in particular, to LASIK.
The ablation profile is calculated, as a rule, prior to implementation of the surgical intervention in respect of the eye to be corrected. The basis of this calculation is a surveying of the eye in its actual state. For this surveying of the eye, the state of the art is familiar with a variety of techniques, in particular topography-measuring instruments (so-called topolyzers), wavefront analysers, Scheimpflug instruments and also pachymeters.
Refractive ophthalmological surgery with a wavefront analyser or with a topography-measuring instrument is hardly capable of resolving small local structures of the cornea within the millimetre range effectively, let alone of assigning them in defined and locally exact manner, in order to enable a locationally faithful treatment. With topolyzers it is also hardly possible to detect so-called central islands within the millimetre range—that is to say, prominences on the cornea which often originate from preceding, not entirely perfect, operations from the early days of PRK.
At present, attempts are being made to track deviations from the desired ablation process in online manner during the ablation, in particular deviations that are based on a so-called cyclotorsion or on a so-called pupil center shift.
But with these processes that are known at present it is, as a rule, not possible to detect local corneal irregularities precisely and to put the laser beam into effect in the course of such a detection in precisely local manner only at this point, and in the process also to track the outcome of the ablation.
So-called optical coherence tomography has been available for some time as a measuring process for non-contacting surveying of biological tissues, cf. for example B. Wolfgang Drexler, Journal of Biomedical Optics, 9 (1), 42-74, 2004. With optical coherence tomography, in particular using broadband irradiators, it is possible to survey very fine biological structures, in particular with resolutions in the region of 1 μm and finer.
EP 1 231 496 A2 describes the application of optical coherence tomography (OCT) for the controlled alteration of tissue in the eye, the treatment laser being controlled as regards power, exposure-time and spot-size. Treated ocular tissue is distinguished therein from untreated ocular tissue by means of OCT and a threshold value. The region of the ocular tissue that has been successfully treated with the laser is determined with OCT.
US 2007/0282313 A1 (Huang et al.) describes the use of OCT only for the purpose of topographical surveying in refractive surgery. No reference is to be found therein to an online-controlled photoablation by means of OCT. In this state of the art the topographical data acquired with OCT are utilised merely for the advance calculation of an ablation program.
EP 0 697 611 A2 describes a system similar to that of EP 1 231 496 A2, discussed above, with an autofocus system for an ophthalmological surgical microscope. Topographical measurements in respect of the cornea are effected therein, but no online control of a resection of tissue with OCT.
US 2007/0073905 A1 does not use OCT but describes generally the state of the art of a surgical intervention in respect of the eye using previous model calculations.
WO 2006/087180 A2 describes a process for ablation, though without using OCT. DE 103 23 422 A1 also does not describe a use of OCT, but only the detection of an optical pressure range in the tissue.